Application should be cross-platform instant messaging service for smartphones with internet access. In addition to text messaging, users should be able to send each other images, video, and audio media messages. The client software should be available for Google Android, Apple iOS and Microsoft Windows Phone, in the first phase.
There should be server side supporting AWS linux instance hosting or Google App Engine Hosting. The server side can be in any one of java, mono, python or php. Message sending could be using GCM / Apple push / Windows push or better would be Amazon SNS push. DB should be the best suitable for the chosen particular environment. For features, Whats App is almost near about what we need, with